Diamond core drills, as with other types of drills, use multiple drill rods to bore into the earth.
Typically, the drill is actuated and drilling commences until a fixed length of drill rod has traveled through a predetermined distance into the ground. At this point, the drilling operation ceases; the drill string connection is broken at a number of locations; the water swivel connections are uncoupled; and an additional drill rod is laboriously inserted into the string at the drill proper. Drilling commences again until the next rod is required.
The stop and start regimen of the drilling cycle leads to downtime and inefficiency. Most drill strings must be manually loaded by at least one operator. The drill must be stopped; the connections broken; water lines disconnected; the new rod carefully threaded to an existing rod; the appropriate connections reattached; and the drill powered up.
Besides being a physically demanding job, the business of loading heavy and clumsy drill rods is dangerous to personnel. Moreover, care must be taken to protect the threads at the ends of the drill rods. Stripping and cross-threading can easily occur due to misalignment and excessive torquing rendering the affected rods useless at the job site.
In particular, diamond core drills use multiple drill rods to drill holes. These drills can operate in any orientation and direction. Vertically upward drilling presents an extra challenge to the diamond driller since the drill string can fall out of the drill hole while rods are being added.
Attempts have been made to automate heavy drill rod loading so as to reduce the possibility of injury to personnel and equipment while increasing productivity.
Essentially an automated rod changer must bring a drill rod, typically four inches (10.2 cm) or greater in diameter with heavy tapered threads, into station, index the existing rod into the proper position reliably and accurately, start the threading process, fully torque the rod into its neighbor and the rotary drill drive and recouple the water lines. Upon completion of the loading cycle, the drill commences drilling until the next rod is required whereupon the stop/load/start cycle is started anew.
Representative designs are taught in U.S. Pat. Nos. 5,791,822 and 5,575,344.
Another drilling technology that utilizes automated drill string decouplers is directional drilling (also called trenchless technology).
This type of drill is used for laying utility piping under highways and buildings without having to disturb the surface. A drill is set up and the rods pushed into the soil at a shallow angle. The bit is angled on the front and can be rotated to send it in a new direction. These drills use two clamps that can be rotated with respect to each other; this allows one to unthread the bottom joint. The rotation units on these drills typically use a top drive head, which means the rod at the head is permanently attached. These systems utilize rods with heavily tapered threads to ensure alignment of the rods during threading operations.
The aforementioned designs are not applicable for small diameter thin walled drill rods.
In particular, wireline diamond core drills that utilize retrievable drill core samples typically employ rods of small diameter, such as AQ size rod. AQ wireline rods are 1.75 inches (4.45 cm) in diameter with a 1.375 inch (3.5 cm) inside diameter. These relatively thin walled rods have light threads that are easily damaged.
In contrast to heavy threaded rods that are somewhat tolerant of initial misalignment and relatively rough handling prior to threading, thin walled rods must be perfectly aligned and lightly torqued prior to engagement. Otherwise the threads will become crossed and stripped.